The protein kinases that constitute mitogen activated protein kinases (MAPK) pathways are currently studied by the pharmaceutical industry because of the central role MAPK modules play in mediating cellular responses to stimuli such as growth factors and cytokines. The MAPK kinases, ERK1 and ERK2, are activated by a phosphorylation signaling cascade in response to hormones and growth factors. Specifically, ERK1 and ERK2 are activated by the kinase MEK1 and MEK2 through dual phosphorylation on conserved threonine and tyrosine residues in the ERK's activation loop. Known oncogenes such as Ras and Raf are upstream activators of ERKs 1 and 2. ERK2 is aberrantly activated in multiple common tumor types, and its inhibition reverses cellular transformation. Hence, there is considerable interest in the role ERK1,2 signaling plays in oncogenic transformation and in targeting ERK1,2 for cancer therapies using small molecules. Structure assisted drug design is a tool used to optimize the success of identifying such therapeutic compounds. However, use of this powerful methodology requires three-dimensional structural information (e.g., as obtained via X-ray diffraction of the target protein). The crystal structure of unphosphorylated ERK2 (Zhang et al., J. Mol. Bio. 233:550-552 (1993); Zhang et al., Nature 367:704-711 (1994); Wang et al., Structure 6(9): 1117-1128 (1998)) and of diphosphorylated ERK2 (Canagarajah et al., Cell 90:859-869 (1997)) was determined. The crystal structure of ERK2 complexed with olomoucine was also determined (Wang et al., Structure 6(9): 1117-1128 (1998)). Nevertheless, there is a need in the art for crystals with which high resolution structural determination can be performed. The present invention addresses this need by providing such crystals.